1. Field of the Invention
This invention relates generally to organic light emitting diodes, and, more particularly, to electrophosphorescent organic light emitting diodes.
2. Description of the Related Art
An organic light-emitting diode (OLED) is a thin-film light-emitting diode that uses an organic compound as an emissive layer. FIG. 1 conceptually illustrates a conventional OLED 100 that includes an emissive layer 105 sandwiched between an anode 110 and a cathode 115. The anode 110 is typically formed of indium tin oxide (ITO) and is used to provide holes 125 to a hole injection layer 130, which may then provide the injected holes 125 to a hole transport layer 135 and then to the emissive layer 105. The cathode 115 is used to provide electrons 140 to an electron transport layer 145 and then to the emissive layer 105. The holes 125 and the electrons 140 in the emissive layer 105 may combine to form excitons 150. The excitons 150 may be formed in either a singlet state (spin 0) or a triplet state (spin 1). The triplet state is more common than the singlet state because approximately 75% of the excitons 150 form in the triplet state, whereas only approximately 25% of the excitons 115 form in the singlet state.
The excitons 150 decay when the hole 125 and the electron 140 combine and release the energy stored in the exciton 150 as heat and/or light 155. In fluorescence OLEDs 100, the emissive layer 105 is formed of materials such that the energy released by singlet excitons 150 is released primarily as light and the energy released by the triplet excitons 150 is released primarily as heat. In contrast, the emissive layer 105 in a phosphorescent OLED 100 is formed of materials such that the energy released by triplet excitons 150 is released primarily as light. Most OLEDs are fluorescent OLEDs, at least in part because fluorescence is generally a faster and more efficient process than phosphorescence. However, phosphorescent OLEDs may be able to operate at a higher overall efficiency, at least in part because of the relatively large ratio of triplet-to-singlet excitons 150.
Conventional phosphorescent OLEDs have a number of drawbacks that have limited their potential usefulness. The organic materials that are used to form the hole injection layer 130 of a conventional phosphorescent OLED are not typically soluble and so must be evaporated onto the surface of the anode 110. For example, the hole injection layer 130 of a conventional phosphorescent OLED may be formed of insoluble small molecules. Depositing materials by evaporation forms a layer of approximately constant thickness over the underlying surface. Consequently, any imperfections in the surface of the anode 110, such as spikes and/or ditches, will also appear on the surface of other layers deposited above the anode 110, such as the hole injection layer 130. Furthermore, the organic materials that are used to form the hole transport layer 135 of a conventional phosphorescent OLED are typically hydrophobic small molecules and so the bond between these layers and hydrophilic layers, such as the surface of the anode 110, may be relatively weak and susceptible to separating when heated.